Coupled circuits



- Sept. 1, 1936. P. o. FARNHAM COUPLED .CIRCUITS Filed March 7, 1935 Patented Sept. `Yl, 1936 TED star-Es ra'rea'r ,g FFICE COUPLED CIRCUITS Y Delaware Application March 7, 1933, serial No. 659,97sf f 17 claims. (o1. iis-44) This invention relates to tuned transmission systems and particularly to methods of and circuit arrangements for coupling tunable circuits such as "are used for the selective reception of radio waves.

It is well known that radio amplifiers embodying simple single tuned circuits in cascade designed to receive signals within a given frequency band by an adjustment of the tuning capacities of the several circuits do not exhibit uniform characteristics asregards amplification and selectivity at all frequencies to which the system may be tuned. The amplication generally rises as the system is tuned to the higher frequencies and at thelsame time the selectivity is decreased. Methods for obtaining both constant amplication and constant selectivity with single tuned circuits are described and claimed in my copending application Ser; No( 564,675, filed Sept. 23, 1931. i Y

It is usually desirable in cascade amplification to use the tuned circuits in pairs for two reasons; (a) the required selectivity far from resonance is obtained without using an excessive number of vacuum tubes, and (ZJ) the response to frequencies near resonance may be made substantially uniform inY order to obtain fidelity in the transmission vof Vthe higher modulation frequencies. The coupling of two tuned circuits materially increases the dimculties presented by the problem of securing uniform gain and selectivity over the tuning band, particularly when each circuit is of the type in which the resistance is varied with the adjustable tuning element to maintain, for the individual circuits, a substantially constant gain and selectivity over the tuning range.

An object of the invention is to provide methods of and circuit arrangements for coupling tunable circuits, which methods and circuit arrangements are adapted to secure desired, and preferably approximately uniform, gain and selectivity characteristics over the entire tuning range. Another object of the invention is to.provideforA use in connection with tunable circuits, coupling methods and circuits which permit the coupling ratio to be maintained at a desired value corresponding either to less than, equal to, or greater than critical shape coupling over the tuning range of the circuits. A further object is to provide, for use With inductance-tuned circuits, coupling methods and circuits` which permitadjustment of the frequency band widthv of the response characteristic to any desired value within wide limits, the band Width in frequency remaining constant over the frequency range for any given adjustment. i

Fig. `1` is a fragmentary circuit diagram of a radio receiver, and illustrating an application of 1 f, the invention to a coupled-circuit radio amplifier stage employing inductance` tuning;

Fig. 2 is a curve, sheet indicating the relative performance o'f 'circuits coupled in'accordance with the invention, and similar circuits coupled by known methods; and 10 Figs. 3 to '7 are cir'cuit diagrams illustrating alternative coupling arrangements.

'I'heV invention applies to coupled circuit systems, either inductance'tuned or'capacity-tuned, which are composed of single circuits constructed in accordance with the disclosures in the aforementioned application; :that is, to systems in which the ratio of inductance to resistance in each of 4the single circuits is maintained substantally constant withrespect totuning. 20

In the drawing, the reference numeral l identifies the tube o f a radio amplifier stage which passes an'amplied radio voltage to the succeeding tube `or other load circuit through the coupled circuits I, Ijeachof which is inductively 25 tuned to resonance at the frequency of a desired signal. 'Ihe tuned circuits lemployed in such coupling system are usually of substantially identical construction and',v for convenience of description and simplicity'of analysis, the circuits 30 I, II are assumed to be of this general type but it` is to beunderstood that the invention is not limited to the coupling ofy two substantially identical circuits. j A

As shown in Fig. l, eachv tuned circuit includes an adjustable inductance 2 for tuning the circuit to resonance, resistance 3 and a relatively fixed condenser 4 for alinement of the circuit. The junction yof condensers 4 is connected to ground through the composite impedance, capacity 5 in series with a small inductance 6. The high potential terminal of circuit I is'connected to the plate of tube l and the corresponding terminal of circuit II may be connected to `the grid of the succeeding amplifier or detector.

The plate supply to the tube I may be completed through the induct'anceand'resistanceof circuit I by connecting the low potential terminal of resistance 3 to ajplate current supply, indicated by -l-EB, and grounding that terminal for radio o frequencies by the condenser 1. Y

We may determine the type of coupling required for two such single circuits when constant gain and selectivity over the tuning range'are desired for the resulting coupled circuits. The

ratio of the coupling coeiilcient to the power ratio of the circuit may be taken as the important f criterion of coupling since it is this quantity which determines the shape of the frequency-response characteristic for a xed tuning adjustment,Y as well as the transmission at resonance For simplicity it will be' assumed that the circuit inductances are equal as over the tuning range.

well as the resistances; in which case the following notation may be employed.

vR=total series resistance of each tuned circuit;

L-finductance of circuit. w=angular frequency of impressed voltage. n=R/wL=circuit power ratio.`

Xm=coupling reactance. lc`Xm/wL=coecient of coupling. K=k/17='coupling ratio. Y Since we have assumed single circuits in which R/L is constant over the tuning range, the power l ratio 1, varies inversely as the first power of the frequency. Thus, in order to make the coupling coupling reactance is so designed as to obtain a coeflicient of coupling that Varies inversely as the first power of the resonant frequency and by this vtuned circuit capacities and power method the desirable result of constant gain and selectivity is obtained over the frequency band covered by the circuits.

v The circuit of Fig. 1 illustrates a specific embodiment of the invention using inductance 'tuning and having variable resistances 3 which satisfy the primary requirement that L/R is constant. In the case of'inductance tuning by means of iron cores movable with respect to the coils, the core design may conveniently be such that the inherentcoil resistance serves as the variable `resistance 3, since the coil resistance varies: with-the adjustment of the cores. By a proper design of the coupling reactance, condenser 5 in series with coil 6, a substantially constant coupling ratio K=7ch7 may be obtained in accordance with the previous discussion. Y Y By selecting values of inductances of coil 6 and capacity of condenser 5 such that the coupling reactancebecomes zero at a frequency somewhat higher than the highest frequency within the tuning range of the circuits, the coupling reactance will be capacitive and Will vary only slightly from its correct value Yover the tuning Yrange of the circuits. The following example is given as illustrating the determination of specific Values of the coupling elements in terms of the ratio for the special case K=1. n

' VLet C'=eiTective capacity ofthe coupling re- Vdetermined by these equations.

Letting C=capacity of the tuned circuit I, Cm`

may be obtained in terms of C andthe powerA ratio a7 of the tuned circuit at, say, 700 kc. where Y Equations (6) and (7) thusgive the required 5 Y values of the coupling elements in terms of th constants of the tuned circuits. v

The curve I0 of Fig. 2 indicates the Vactual operation of the circuit whenV theY coupling' reactances have values corresponding to .those Y The curve lJ is plotted between frequency and values of C, v Y

where C is the effective capacity of the coupling reactance and C is the capacity which is required to satisfy the desired condition thatthe coupling ratio K remain constant and, under the particular conditions assumed, equal to 1. K

' It is obvious that the desired constant'gain and selectivity characteristics are obtained only for those frequencies at which the actual coupling capacity C is equal to'there'quired value C", and that there is a departure kfrom constant gain and selectivity at frequencies for whichrthe ratio of the actual to the desired capacityis not unity. Curve I0 shows that the desired conditions were satisfied at 700 to 1400 kilocycles, i. e., at the frequenciesassumed for 1 and 2 incomputation of the coupling reactances, and that the Ymaxi.- mum departure of the actual capacity VC from Vthe desired value, and hence the actual departure value when circuits I and II are coupled in the known manner by .a simple capacitive coupling,

i. e.; when the corrective inductance 6 isomitted. Curve I I was plotted for a value of C'=Cm which Vwas equal to the desired value C" at 700 kilocycles. Curve Il shows that the coupling at higher frequencies departs from the desired value by an amount considerably greater than that exhibited vby a coupling circuit embodying the invention. Curve I2 indicates the performance of the coupled circuits when the value of a simple .of the coupling reactance to the shaft or adjustable tuning member.

As shown in Fig. 3, the tuned circuits I, II may each include a relatively fixed capacity C, a tuning inductance L and a variable resistance R; the inductance and resistance of both circuits being simultaneously adjusted, as indicated by broken lines 9 which represent a mechanical interconnection of the adjustable elements, to maintain a constant gain and selectivity for the individual circuits. A variable coupling condenser Cm is connected between the high potential terminals of the circuits, and the adjustable element ofthe variable condenser is connected to the gang control 9 through a mechanical connection 9. With Vthis arrangement, the adjustment of C'm should be such that the coupling capacity varies inversely as the frequency. The input terminals ofthe rst circuit are illustrated as connected vto a conventional antenna structure A through the usual coupling condenser C0, and to ground G, respectively. It is to be understood that each of the coupling systems herein illustrated may be employed as an interstage coupling between vacuum tubes, as a coupling between a collector structure and the rst tube ofV a receiver system, or between other types of sections of a tuned alternating current transmission system.

As shown in Fig. 4, the tuned circuits I, II may lbe coupled by a variable condenser Cm which is connected between the ground terminals of the circuits and their respective Yhigh potential terminals through the condensers C2.. llhe cou,- pli-ng ooeiicient is dependent upon the relative `values of C2 and Cm and, by mechanically connecting the coupling condenser to the tuning controls, Cm may be varied with frequency to obtain desired gain and selectivity characteristics. For constant gain and selectivity, Cm should vary directly with the frequency to keep K constant.

Mutual inductance M may be employed for coupling the circuits, as shown in Fig. 5, by so connecting the adjusting elements of the coupled coils I3 with the tuning controls that the in,- ductance M varies inversely as the cube of the frequency when K is to remain constant.Y

The invention provides. a simple and: ecient method of obtaining substantially constant gain and selectivity when each of the coupled circuits is tuned by adjusting an iron core axially of the inductance. includes a coil L shunting a relatively fixed tuning condenser C, and tuned by the adjustment As shown in Fig. 6, each circuitA of an iron core I4. The resistances R which are shown as a series velement of each circuit are not physical resistances, but the inherent resistances of the circuit which, as is well known, vary automatically with the tuning adjustment of the i cores, and at such rate as to impart substantially constant gain and selectivity characteristics to the individual circuits.' The low potential terminals of the circuit are connected to ground through the coupling condenser Cm, and the cores are grounded, as indicated by leads l5.

The Aeffective capacities C3 between the coils and the cores serve to couple the circuits through the common capacity Cm. Capacities C3 vary with the tuning and, as the cores are inserted into the coils to increase the circuit inductance for lower frequencies, these effective capacities increase to provide the tighter coupling between the circuits necessary to maintain K constant. The advantage of this system lies in the fact that all elements necessary for varying the coupling co-efcient in the proper manner are already provided in the structure of the tuning elements.

Although the system does not include mechanical connecting elements additional to those providing the gang tuning connection of the cores I4, as indicated by broken line 9, dotted lines I6 are shown as connecting the adjustable members of the resistances R and capacities C3 to the gang control, thus indicating graphically that the values of these elements vary automatically with the tuning.

The circuit shown in Fig. 6 lends itself readily to an adjustment of the shape or useful band width` of the selectivity curve. If the values are so chosen that the coupling ratio K is constant over the tuning range for one value of the coupling capacity Cm, the coupling ratio may be adjusted to a different but frequency-independent value by adjustment of the capacity Cm. The shape of the selectivity curve may therefore be changed to secure greater iidelity of transmitted side band frequencies, whenever local interference conditions permit, by adjusting Cm to a smaller value to obtain greater coupling corresponding to a higher value of the coupling ratio K. The value of K may be adjusted to correspond to critical coupling or to either less than or more than critical coupling, and the selected type of coupling will be maintained over the entire tuning range.

The inductances of the circuits shown in Figs. 1, 3, 4 and 5, may, of course, be varied by the adjustment of iron cores or by other means. When iron cores are employed, the resistances R are not physical elements but are the inherent resistances of the circuits.

The invention is equally useful with condenser tuned circuits' and Fig.. 'Z shows a condenser tuned system using a coupling similar to that of the inductance. tuned circuits of Fig. 3. Each circuit includes an inductance L, a tuning condenser C and a physical' resistance R' which is mechanically connected to the tuningY control, as indicated by broken line 9. The design and frequency adjustment of the resistance R' is such that the total series resistance R of the circuit varies to maintain L/R constant. The adjustablel element of the coupling condenser Cm is varied with tuning by a mechanical connection 9 to the tuning control. To maintain4 a constant coupling ratio K, the value of condenser C'm must vary inversely asV the cube' oi the frequency as the circuits are tuned over the frequency band.

The invention therefore provides a methodof Y and selectivity over the tuning range, -to obtain constant or substantially constant overallVY gain Yand selectivity. As indicated in the above vdiscussion of the illustrated types of coupling systems, the exact variation of the coupling co-eicient with frequency will depend uponthe. particularcoupling method employed but. with anyY given type of coupling, the principles stated above will. enable those 'familiar with this art to design the coupling reactance to providethe desired gain and frequency characteristics.

I claim: 1

l. InV the operation of a transmission system including two tunable circuits coupledby a reactive coupling, the method of controlling the overall gain and selectivity characteristics cf the coupled circuits which comprises tuning each circuit to resonance at the desired frequency `of transmission, varying the power ratio of Veach ltuned circuit as an inverse function of Vthe' frequency, and vsimultaneously varying thefreactance of the reactive vcoupling substantially as an even power of the frequency to which'the circuits are tuned.

'2. In the operation of a transmission Ysystem including two tunablercircuits coupledjby a re'- active coupling and each circuit including resistance, Vthe method of controlling the overall gain and selectivity characteristics Aof the `coupled circuits which'comprises adjusting each circuit to resonance at the desired frequency of transmission, adjusting the resistance of each circuit simultaneously with tuning to maintain desired gainV and selectivity characteristicsrfor each circuit as it istuned over its frequency range, and simultaneously varying the reactance of the reactive coupling substantially as an even power of the frequency to which the circuits are tuned.

3. In the transmission of alternating currents Vby coupled circuits each of which Yis tunable Yover a frequency range and includes resistance, the vmethod which comprises tuning each circuit tothe frequency of the currents to beV transmitted, Varying the resistance of each circuit with tuning to maintain approximately constant gain and selectivity characteristics for the separate circuits, coupling the circuits with a reactive coupling to obtain a desired coupling ratio at one transmission frequency, andfvarying the coupling coefcient with tuning and substantially inversely as the Vfirst power vDf .the

Yfrequency toV which thev circuits are tuned.`

4.' In the transmission of alternating currents` by coupled circuits each of Y.which is tunable over a frequency range and includes resistance, the

Vmethodcwhich comprises tuning each circuitto the frequency of the currents to be. transmitted, varying the resistance of each circuit with tuning to maintain approximately constant gain and selectivity characteristics for the separate circuits, coupling the circuits with areactive coupling to obtain aV desired coupling ratioV at one transmission frequency, and varying the Vreact.- ance of the reactive coupling with tuning and substantially as an even power of the frequency to which the circuits are tuned. Y .Y l

5. In an electrical transmission system, the combination with two circuits serially arranged in the` direction of transmission and eachY tunable over a frequency-band, means for tuning l:apesar-o3 said circuits simultaneously tor the desiredi'frequ'encyv of: maximumA transmission within Vsaid frequency band,`said last means comprising a Vreactive coupling between said circuits.

6. AY transmission system as claimed 5in' claim v5, wherein said reactive coupling. comprises" an.y

inductan'c'el-'capacity 'network Vhaving a capacitive reactanc'e throughout the tuning range-ofi Y said circuits.

7; rA transmission system as claimed in claim 5', whereinisaid Y reactive coupling Y comprises a complex reactance having, throughout the tuning range'of said'circuits, acapacitive reactance highest frequencyV said 4tuning range.V

8. In 'an electrical transmission system, two circuits tunedl to substantially the same freof 'the 'twoY circuits being adjustableV to tune the'circuits over aY frequency band, reactive coupling between sai-d circuits, means operative upon -quency, each circuit including resistance and reactances offopposite signs, similar reactancesrf25 adjustment of Vsaid similar VreactancesV to vary f theV power ratiosY of said circuits to suppress variation of the gain and selectivity-characteristics ofthe separate circuits, and means forV sov varying Vthe'coeilicient of coupling Vwith tuning and substantiallyfinversely as the first power of the frequency to which the circuits are tuned. 9. An electrical transmission systemY as set forth in claim 8,"wherein a mechanical'connection iswprovided between the adjustable reactance of said circuits for simultaneously adjusting the samerto tune said circuits, and'rsaid reactive coupling includes a reactance Vhaving an adjustable element mechanically connected to said mechanical connection. Y v 10.1A transmissionsystem as set forthjin claim 8, whereinthesaid similar adjustable reactances of said-circuitscomprise inductances and grounded iron cores adjustablewithrrespect thereto to `tune said circuits, 'and said reactive coupling includes a capacity common to said circuits andV in series with Vthe inherent capacities between the cores and inductancesV ofY the'separate cir- U .tics for said cascaded circuits, Ysaid lastrmeans including means for varyingnthe coefficient of coupling between said circuits with tuning and approximately inversely as the rst power ofV theV frequency to which Vsaid circuits are tuned. v

12. In an electrical transmission system,the

combination with two circuitseach tunable` over a frequency band and each having'approximately constant gain and selectivitycharacteristics, of

adjustable means reactively coupling said circuits in cascade, said couplingl means being manually adjustable to Vobtain critical shape coupling or either more or less-than critical coupling between said circuits when Ythe latter are simultaneously tuned to the same frequency, andmeans Y for automatically varying the reactance of said Yth-at approaches 'Zero ata frequency Vvabove the V270.V

coupling means with frequency to maintain throughout the tuning range that shape of coupling determined by the manual adjustment made at one frequency and simultaneously therewith to maintain approximately constant overall gain and selectivity characteristics for said cascaded circuits.

13. In an electrical transmission system, the combination with' two tuned circuits each including resistance and reactances of opposite types, and means for simultaneously adjusting one reactance of each circuit to maintain said circuits continuously in resonance when tuned over a frequency band, of means for adjusting the resistance of each circuit with tuning to maintain a constant ratio between the resistance and inductance'of the individual circuits, means reactively coupling said circuits, and means automatically varying the reactance of said coupling means with frequency to maintain a substantially constant coupling ratio throughout the frequency range, said automatic means varying the coupling coeicient approximately inversely as the first power of the frequency to which said circuits are tuned.

14. An electrical transmission system comprising a pair of circuits including substantially identical inductances shunted by substantially identical capacities, iron cores adjustable with respect to said inductances, means connecting said cores for simultaneous adjustment to maintain said circuits continuously in resonance when tuned over a frequency band, means grounding said cores, and a common capacity connected between ground and the low potential terminals of said circuits, whereby said circuits are coupled through said common capacity and the capacities between the cores and inductances of the individual circuits.

15. In an electrical transmission system the combination with two circuits serially arranged in the direction of transmission, each of said circuits being tunable over a frequency band, means for tuning said circuits simultaneously to the desired frequency within said band and means operative with said tuning means for suppressing variations of the gain and selectivity characteristics of the separate circuits, of means for suppressing Variations of the overall gain and selectivity characteristics of said circuits as the latter are tuned over the frequency band, said last-named means comprising a reactive coupling between said circuits arranged so that the coupling reactance thereof becomes zero at a frequency which is somewhat higher than the highest frequency of the tuning range.

16. In an electrical transmission system the combination with two circuits serially arranged in the direction of transmission each of said circuits being tunable over a frequency band, means for tuning said circuits simultaneously to the desired frequency within said band and means operative with said tuning means for suppressing variations of the gain and selectivity characteristics of the separate circuits, of means for suppressing variations of the overall gain and selectivty characteristics of said circuits as the latter are vtuned over the frequency band, said last named means comprising a reactive coupling between said circuits variable simultaneously with the variations in tuning of said circuits.

17. A transmission system as claimed in claim 16 wherein said reactive coupling comprises a capacity arranged with respect to the tuning means of said circuits so that the capacity value thereof varies inversely with the frequency to which the tuning circuits are tuned.

PAUL O. FARNI-IAM. 

